1. Field of the Invention
This invention relates generally to a system and a process for forming a filter medium into a predetermined shape at high speed and with high accuracy, and more particularly to such a system and a process for preparing a filter medium for a filter element of the type in which pleats radially outwardly extend so that fluid to be filtered flows generally radially inwardly along the length of each pleat.
2. Description of the Prior Art
Sheet-type filter mediums such as filter papers have been used to separate and remove sold particles from liquid or gas in a variety of technical fields. Such filter mediums have been formed of various materials and formed into various shapes in accordance with requirements. In general, industrial filter mediums are formed into predetermined shapes and thereafter housed within casings to be used as filters. Such filters are required to uniformly filter fluid throughout the whole sections and to be high in filtering performance. For example, in an air filter for an automotive internal combustion engine, the filter medium such as filter paper is usually pleated and required to always maintain a predetermined shape as a filter element in order to exhibit a high filtering performance.
In conventional filter elements, when they are disposed within the casings after produced, they unavoidably slightly expand owing to their own elasticity, in which a part of the pleats formed in the filter medium expands. Accordingly a part of the filter medium becomes larger in distance between the adjacent pleats while another part becomes smaller. As a result, flow of fluid to flow through the filter element becomes ununiform.
In case of producing a small-sized filter to meet the requirement for a compact filter, it is fundamentally sufficient to make smaller the filter element including the filter medium; however, filtering area is unavoidably reduced merely by minimizing the filter element and the filter paper, thereby lowering filtering performance. In this regard, if a filter element including a multi-layered filter mediums is used for the purpose of obtain a compact filter having a larger filtering area, pressure differential increases between the upstream and downstream sides of the filter element, which requires applying a high pressure to the fluid to pass through the filter element. Additionally, this requires a large-sized pump to circulate the fluid through the filter element.
Accordingly in order to obtain a compact and high filtering performance filter, it is necessary to form many pleats in the filter medium to increase the filtering area. In this connection the shape and the distance of the pleats of the filter medium are determined in accordance with the sizes and the characters of solid particles to be filtered.
A conventional filter medium forming system to form a filter medium having many pleats is, for example, disclosed in Japanese Patent Publication No. 57-7806. In this filter medium forming system, score or bending lines are formed on an elongate filter medium by pressing two score line forming rollers onto the filter medium under the pressing action of two press rollers. Subsequently, the filter medium is flattened by two drive rollers and thereafter is compulsorily carried forward through a converging guide passage in which the filter medium is brought into frictional contact with a plurality of guide ribs whose height increases in the advancing direction of the filter medium.
However, the following difficulties have been encountered in this conventional filter medium forming system: When the filter medium is flattened by the drive rollers, the score or bending lines tend to be removed owing to elasticity of the filter medium so that the surface of the filter medium becomes flat. This increases resistance against bending or folding the filter medium along the score lines, thereby causing accidents in which the filter medium is compulsorily bent or folded at portions other than the score lines. Additionally, since the filter medium is compulsorily carried upon being in frictional contact with the guide ribs, the filter medium is caught by the guide passage, i.e., causing jamming of the filter medium during carrying of the filter medium particularly in case the filter medium is not folded at the right score lines, so that the filter medium cannot be smoothly carried. In such an event, the filter medium forming system must be stopped to remove the jammed filter paper and to adjust various parts of the system, and thereafter the system is restarted for the first time. Such troubleshooting requires a relatively long time. Additionally, the filter medium got off from the system partially expands owing to its own elasticity, so that the distances between pleats become ununiform as a whole.
Furthermore, the pleated filter medium folded at portions other than the right score lines are discarded as rejects in connection with quality control of products, thereby lowering yield rate of products. Thus, it has been eagerly desired to rightly and securely fold the filter medium at predetermined patterns thereby to form a pleated filter medium having a large number of pleats, at a high speed and with a high accuracy.
Moreover, since the filter element formed of the filter medium prepared by the above conventional filter medium forming system is so constructed as to maintain the shape of the filter element by the strength of the filter medium itself, the filter element tends to readily deform. Accordingly, the filter element is inconvenient in handling and has possibility of deforming in use. Additionally, the filter element expands when disposed within the casing after thus formed. In this state, the pleats formed in the filter medium also partially extend, thus forming a part having a larger distance between the pleats and another part having a smaller distance between the pleats. As a result, flow of fluid to pass through the filter medium unavoidably becomes ununiform.
U.S. Pat. No. 4,710,297 issued on Dec. 1, 1987 to Mitsutoshi Suzuki et al discloses a fluid filter including a filter element whose pleated filter medium has a plurality of radially outwardly extending pleats. Two sides or legs of each pleat are bonded to each other at the outer peripheral side to form an outer bonded section while one sides or legs of the adjacent pleats are bonded to each other at the inner peripheral side to form an inner bonded section, thereby forming a fluid path extending between the adjacent pleats along the length of each pleat. Another fluid path is of course formed from the upper side to the lower side of the filter medium. Thus, two kinds of fluid paths (or radial and vertical fluid paths) are formed. This filter element has a broader area through which fluid to be filtered passes. Additionally, fluid flows along the length of the pleats on the upper side of the filter medium and obliquely through the filter medium. This prevents abrupt change in flow direction of fluid to be filtered, thereby lowering flow resistance of fluid to be filtered.
In order to produce such a filter element, the filter medium is formed into a predetermined annular shape after adhesive is applied, taking account of the characteristics of the filter medium and the folding structure of the filter medium. However, an effective system for forming the filter medium with adhesive into a predetermined shape has hitherto not yet been proposed, therefore it has been impossible to automatically produce the filter element at high speed and with high precision.